Is it possible to work on two or more files at the same time (or open multiple Blender instances?)
On systems other than OSX (Windows, Linux, BSD.. etc), you can open as many instance of Blender as you like. (and load whatever file you want)

On OS X, by default, only 1 instance of an app can be run. You can work around this limitation by invoking Blender from the command line. open -n /Applications/blender.app